This invention relates to exhaust gas purification arrangements containing both a catalyzer and a gas adsorber.
German Offenlegungsschrift No. 40 33 827 and European Patent Application No. 0 532 803 describe exhaust gas purification arrangements having both a catalyst and a gas adsorber which are especially suited for motor vehicle internal combustion engines. These documents also disclose information relating to the choice of a gas adsorber, which is generally a zeolite. However, the term "adsorber" as used herein, means any arrangement having a gas adsorbent action and, at higher temperatures, a desorbent action.
Providing an adsorber for the temporary retention of hydrocarbons contained in exhaust gases takes into account the fact that a catalyzer initiates its exhaust gas-purifying action only after it has reached its start-up temperature, which, depending on the age of the catalyzer material, is in the range of 250.degree.C. to 350.degree.C. Consequently, following cold-start of an internal combustion engine, the exhaust gases leave the exhaust system in an unpurified state, resulting in environmental problems, unless corrective action is taken.
It is known, for example, from German Offenlegungsschrift Nos. 1 299 005 and 42 08 624 and from German Engineering Association News (VDI-Nachrichten), No.38 of Sep. 24, 1993, page 11, to heat a catalyzer, either electrically or with the aid of a combustion chamber, so that it reaches its operating temperature more rapidly. As a rule, however, such arrangements do not permanently provide more rapid heating to a desired temperature within an acceptable time period.
In conventional arrangements for preheating catalysts within the required time period by heating the catalyzer to its operating temperature with hot exhaust gases, it has been assumed that the catalyzer will have reached its operating temperature, at least in significant regions, before a gas adsorber preceding the catalyst has been heated by the hot exhaust gases to its gas desorption temperature. At the gas desorption temperature, the adsorber releases the stored exhaust gases for catalytic treatment in the catalyzer. The gas-release requirement, however, which must be met under all age conditions of the catalyzer, is further complicated by another problem, particularly important in the case of motor vehicle internal combustion engines which, as is known, must be operated over a very wide range of ambient temperatures, down to -20.degree.C. or less. This problem is that, in the case of very low ambient temperatures and short trips by the motor vehicle, the adsorber may not release the stored exhaust gases during such short operating times because cause it does not reach its desorption temperature. Consequently, the adsorber will not be capable of adsorbing and retaining newly-generated quantities of exhaust gas components.